Hydrochloric Acid Decomposition of the Niobium–Rare-Earth Slag Produced by Reducing Roasting of the Rare-Earth Ore from the Chuktukon Deposit

The results of studying the hydrochloric acid decomposition of the niobium–rare-earth slag produced by reducing roasting of the high-iron rare-earth ore from the Chuktukon deposit are discussed. The slag is presented by four main phases: a glassy phase, a phase with a perovskite structure, a phase with a loparite structure, and a MnAl2O4-based spinel phase. Niobium and rare-earth metals in the slag are distributed between the first three phases. The hydrochloric acid leaching of the slag is carried out in two stages: leaching under atmospheric pressure and the solid residue is then leached in an autoclave at high temperatures. The slag starts to decompose at very low acid concentrations (pH 4–2.5), but the maximum development of slag (70%) is reached for leaching with 20% HCl at ~100°C. Under these conditions, only the glassy phase of the slag decomposes. The phases with loparite and perovskite structures decompose at high temperatures in an autoclave. The completeness of their decomposition is achieved at 20% HCl and 200°C for 2 h. According to X-ray diffraction analysis data, the spinel phase weakly decomposes by pressure leaching and remains in the solid phase. The yield of the solid phase is ~12% of the slag weight. Niobium, titanium, and zirconium are nearly completely concentrated in the form of oxides in the solid phase along with the spinel phase. The spinel phase can be removed from the solid residue using magnetic separation. The collective niobium–titanium–zirconium concentrate isolated to a nonmagnetic fraction can further be processed using the chloric method to produce the corresponding metals. The hydrochloric solution formed upon pressure leaching is proposed to be directed to the first leaching stage of the slag at ~100°C. This procedure makes it possible to decrease the hydrochloric acid consumption during leaching to the maximum extent and to substantially facilitate the further recovery of rare-earth metals and manganese by precipitation from weakly acidic solutions.